The lowest price component in this Seat (model W/x) parts-list fiche is the Screw.
Parts such as the Seat (model W/x): Bracket,seat Lock, Cushion,rider Hndl(17x24x1.5), Cushion,rider Hndl(17x24x1.5), Handle,pillion Rider are included in the Gsx600f Katana 1999 (x) Usa (e03) Seat (model W/x) parts fiche.
In this schematic the "Cushion,seat(40x20x15)" is used most often.